Wind-electric generators provide a clean and renewable source of electric energy. Mounting wind-electric generators offshore is particularly attractive, as winds offshore are often stronger and steadier than on land. Further, offshore structures often face less resistance by neighbors afraid of noise pollution or the appearance of high rising structures in general. However, wind-electric generators experience strong forces acting on their rotor—the forces stemming from the impulse-reduction of air mass flowing through the rotor. These forces act on the rotor shaft, which is mounted highly elevated on a mast. The resulting lever effect of wind force acting on a high mast causes significant torque, which has to be absorbed by an offshore mounting system carrying the wind-electric generator.
Several approaches for mounting wind-electric generators offshore are known. Wind-electric generators can stand directly on the sea bed. A monopile-construction consists of a pipe, which is driven into the sea bed and continues to the mast. Heavy weight grounding consists of a heavy foundation that is placed on the sea bed and to which the mast is attached. Tripod structures exist, which distribute the forces acting on the mast onto several anchors that are grounded in the sea bed. Common to all these approaches is that the mounting system may oscillate when excited by wind and waves. In the worst case, the system may be excited at its resonance frequency, causing unfavorable stress on its parts. These mounting solutions are also limited to shallow waters. A detailed description of such constructions can be found in Martin Kuhn, “Dynamics and Design Optimisation of Offshore Wind Energy Conversion Systems,” DUWIND Delft University Wind Energy Research Institute, Report 2001.002.
Wind-electric generators may also be attached to mounting systems, which use buoyancy-force to carry the generator's weight. In such systems a carrier having a buoyancy volume is tied to the ground with ropes that are pretensioned by excess buoyancy, i.e. where the buoyancy force acting on the system is larger than the system's weight. The ropes may be kept short so that the whole buoyancy volume is kept under water. Keeping the whole buoyancy volume under water maximizes the buoyancy force and causes high pretension force on the ropes. The center of buoyancy in these systems is far below their center of gravity. The described mounting systems may allow some horizontal motion of the carried structure. Excess buoyancy can be dimensioned to control the self-frequency of the system, so that there is only low excitation with the motion of the waves. By allowing horizontal motion resulting forces can be reduced. By using the right amount of buoyancy, the motion of the mounting system can be limited to about one meter. To compensate high torque acting on the carried structure, either the pretension of the ropes has to be very high or the attachment points of the ropes at the carrier have to be far apart, which also reduces the forces acting on anchors used for attaching the ropes to the sea bed.
These mounting systems do not effectively limit motion of the mounting platform around the yawing axis, however. The mounting platform and attached structure will therefore have the tendency to make a yawing motion, which has to be compensated by forces. In order to reduce the yawing motion at wind-electric generators, special wind-electric generators are used, where the rotor is mounted closer to the mast. Buoyancy based mounting systems can be used in deep water; however the cost of the rope is rising and the restoring force gets lower, if the excess buoyancy is the same. By using ropes to attach the carrier to the anchors in the sea bed, only pulling forces act on the anchors. This leaves the anchors' potential of transferring pushing forces into the sea bed unused.
Semi-submersible platforms also exist, to which several wind-electric generators are stiffly attached. Cylindrical buoyancy tanks are attached to the platform to provide floating stability. The semi-submersible platform is held in position with chain cables, so that motion in all directions is possible. Due to the large size of the semi-submersible platform, torque, that results from forces acting on the wind generators mounted to the platform, can be absorbed. This design can be used in deep water. Unfortunately, the generator masts in these systems are again sensitive to excitation by waves.
Further known are mounting systems that consist of a tube float mounted on one end with a cardan joint to the sea bed. A platform can be mounted on the top of the tube float. The tube float is kept in an upright position by excess buoyancy. In these types of construction, the relation between the lever arm above the water and the lever arm below the water is decisive. The longer the lever arm above the water relative to the lever arm below the water, the less efficient the construction becomes. A description of such a construction can be found in Peter Wagner's “Meerestechnik,” edited by Ernst & Sohn, Berlin 1990, chapter 6.3.2.
In light of the problems associated with existing approaches, improved systems are needed for mounting wind-electric generators and other high rising structures offshore. The mounting system should be compact in size and inexpensive to make. The mounting system must be capable of absorbing the high wind forces that act on the elevated rotor while at the same time limiting the undesirable motion caused by waves.